Peripheral Acuity

With strict fixation of the center spot, each letter is equally legible because it is about ten times its threshold size. This is true at any viewing distance. Chart shows the increasingly coarse grain of the retinal periphery. Each letter is viewed by an equal area of visual cortex (“cortical magnification factor”) (Anstis, S.M., Vision Research 1974).  

The left hand picture shows the San Diego skyline. The right hand picture is progressively blurred from the center to the periphery. When fixated at their respective centers, both pictures look equally sharp because the progressive blurring in the right hand picture just matches the progressive loss of acuity with eccentricity caused by the increasingly coarse grain of the peripheral retina.

The retinal image undergoes barrel distortion in the retinal ganglion layer and in the visual cortex V1. This barrel distortion, or greater magnification of the center than the periphery, reflects the “cortical magnification factor”.